SYNOPSIS

DESCRIPTION

Packet sockets are used to receive or send raw packets at the device driver
(OSI Layer 2) level.
They allow the user to implement protocol modules in user space
on top of the physical layer.

The
socket_type is either
SOCK_RAW for raw packets including the link level header or
SOCK_DGRAM for cooked packets with the link level header removed.
The link level
header information is available in a common format in a
sockaddr_ll. protocol is the IEEE 802.3 protocol number in network order.
See the
<linux/if_ether.h> include file for a list of allowed protocols.
When protocol
is set to
htons(ETH_P_ALL) then all protocols are received.
All incoming packets of that protocol type will be passed to the packet
socket before they are passed to the protocols implemented in the kernel.

Only processes with effective UID 0 or the
CAP_NET_RAW capability may open packet sockets.

SOCK_RAW packets are passed to and from the device driver without any changes in
the packet data.
When receiving a packet, the address is still parsed and
passed in a standard
sockaddr_ll address structure.
When transmitting a packet, the user supplied buffer
should contain the physical layer header.
That packet is then
queued unmodified to the network driver of the interface defined by the
destination address.
Some device drivers always add other headers.
SOCK_RAW is similar to but not compatible with the obsolete
AF_INET/SOCK_PACKET of Linux 2.0.

SOCK_DGRAM operates on a slightly higher level.
The physical header is removed before the packet is passed to the user.
Packets sent through a
SOCK_DGRAM packet socket get a suitable physical layer header based on the
information in the
sockaddr_ll destination address before they are queued.

By default all packets of the specified protocol type
are passed to a packet socket.
To only get packets from a specific interface use
bind(2)
specifying an address in a
struct sockaddr_ll to bind the packet socket to an interface.
Only the
sll_protocol and the
sll_ifindex address fields are used for purposes of binding.

sll_protocol is the standard ethernet protocol type in network order as defined
in the
<linux/if_ether.h> include file.
It defaults to the sockets protocol.
sll_ifindex is the interface index of the interface
(see
netdevice(7));
0 matches any interface (only permitted for binding).
sll_hatype is a ARP type as defined in the
<linux/if_arp.h> include file.
sll_pkttype contains the packet type.
Valid types are
PACKET_HOST for a packet addressed to the local host,
PACKET_BROADCAST for a physical layer broadcast packet,
PACKET_MULTICAST for a packet sent to a physical layer multicast address,
PACKET_OTHERHOST for a packet to some other host that has been caught by a device driver
in promiscuous mode, and
PACKET_OUTGOING for a packet originated from the local host that is looped back to a packet
socket.
These types make only sense for receiving.
sll_addr and
sll_halen contain the physical layer (e.g., IEEE 802.3) address and its length.
The exact interpretation depends on the device.

When you send packets it is enough to specify
sll_family, sll_addr, sll_halen, sll_ifindex. The other fields should be 0.
sll_hatype and
sll_pkttype are set on received packets for your information.
For bind only
sll_protocol and
sll_ifindex are used.

Socket Options

Packet sockets can be used to configure physical layer multicasting
and promiscuous mode.
It works by calling
setsockopt(2)
on a packet socket for
SOL_PACKET and one of the options
PACKET_ADD_MEMBERSHIP to add a binding or
PACKET_DROP_MEMBERSHIP to drop it.
They both expect a
packet_mreq structure as argument:

mr_ifindex contains the interface index for the interface whose status
should be changed.
The
mr_type parameter specifies which action to perform.
PACKET_MR_PROMISC enables receiving all packets on a shared medium (often known as
"promiscuous mode"),
PACKET_MR_MULTICAST binds the socket to the physical layer multicast group specified in
mr_address and
mr_alen, and
PACKET_MR_ALLMULTI sets the socket up to receive all multicast packets arriving at
the interface.

In addition the traditional ioctls
SIOCSIFFLAGS, SIOCADDMULTI, SIOCDELMULTI can be used for the same purpose.

Ioctls

SIOCGSTAMP can be used to receive the timestamp of the last received packet.
Argument is a
struct timeval.

NOTES

For portable programs it is suggested to use
AF_PACKET via
pcap(3);
although this only covers a subset of the
AF_PACKET features.

The
SOCK_DGRAM packet sockets make no attempt to create or parse the IEEE 802.2 LLC
header for a IEEE 802.3 frame.
When
ETH_P_802_3 is specified as protocol for sending the kernel creates the
802.3 frame and fills out the length field; the user has to supply the LLC
header to get a fully conforming packet.
Incoming 802.3 packets are not multiplexed on the DSAP/SSAP protocol
fields; instead they are supplied to the user as protocol
ETH_P_802_2 with the LLC header prepended.
It is thus not possible to bind to
ETH_P_802_3; bind to
ETH_P_802_2 instead and do the protocol multiplex yourself.
The default for sending is the standard Ethernet DIX
encapsulation with the protocol filled in.

Packet sockets are not subject to the input or output firewall chains.

Compatibility

In Linux 2.0, the only way to get a packet socket was by calling
socket(AF_INET, SOCK_PACKET, protocol). This is still supported but strongly deprecated.
The main difference between the two methods is that
SOCK_PACKET uses the old
struct sockaddr_pkt to specify an interface, which doesnt provide physical layer
independence.